WASHINGTON — As the proposed agreement over Iran’s nuclear program is debated in coming weeks, President Obama will make his case to a Congress controlled by Republicans who are more fervently pro-Israel than ever, partly a result of ideology, but also a product of a surge in donations and campaign spending on their behalf by a small group of wealthy donors.

One of the surprisingly high-profile critics is Senator Tom Cotton of Arkansas, who burst to prominence with a letter signed by 46 Republican colleagues to leaders of Iran warning against a deal. Mr. Cotton, echoing criticism by Israeli leaders, swiftly denounced the framework reached on Thursday as “a list of dangerous U.S. concessions that will put Iran on the path to nuclear weapons” — words, his colleagues say, that expressed his deep concern about Iran’s threat to Israel’s security.

But it is also true that Mr. Cotton and other Republicans benefited from millions in campaign spending in 2014 by several pro-Israel Republican billionaires and other influential American donors who helped them topple Democratic opponents.

Republicans currently in the Senate raised more money during the 2014 election cycle in direct, federally regulated campaign contributions from individuals and political action committees deemed pro-Israel than their Democratic counterparts, according to data compiled by the Center for Responsive Politics and analyzed for The New York Times by a second nonprofit, MapLight. The Republican advantage was the first in more than a decade.

The alliances in Congress that pro-Israel donors have built will certainly be tested as they lobby lawmakers to oppose the deal with Iran and perhaps even expand sanctions against the country, despite objections from the Obama administration.

Donors say the trend toward Republicans among wealthy, hawkish contributors is at least partly responsible for inspiring stronger support for Israel among party lawmakers who already had pro-Israel views.

“Absolutely, it is a factor,” said Marc Felgoise, who manages the Philadelphia Israel Network, a campaign fund-raising group, and whose own contributions have shifted to Republicans, though he still supports many Democrats. “They are trying to cater to people who are ultimately going to support them.”

Senator Lindsey Graham, Republican of South Carolina, saw his donations from pro-Israel donors soar to about $285,000 in the 2014 election cycle from less than $100,000 in 2008, during his previous election, the analysis by MapLight shows. Pro-Israel contributions to Senator Richard J. Durbin, Democrat of Illinois, plummeted to less than $150,000 in 2014, when he was also re-elected, from nearly $300,000 in 2008, according to this count.

But few candidates have benefited as much as Mr. Cotton.

The Emergency Committee for Israel, led by William Kristol, editor of the conservative Weekly Standard, spent $960,000 to support Mr. Cotton. In that same race, Paul Singer, a hedge fund billionaire from New York and a leading donor to pro-Israel causes, contributed $250,000 to Arkansas Horizon, another independent expenditure group supporting Mr. Cotton. Seth Klarman, a Boston-based pro-Israel billionaire, contributed $100,000.

The political action committee run by John Bolton, the United Nations ambassador under President George W. Bush and an outspoken supporter of Israel, spent at least $825,000 to support Mr. Cotton. That PAC is in part financed by other major pro-Israel donors, including Irving and Cherna Moskowitz of Miami, who contributed 99 percent of their $1.1 million in 2012 races to Republican candidates and causes.

Jeremy Ben-Ami, president of J Street, a liberal pro-Israel group, said this relatively small group of very wealthy Jewish-Americans distorted the views among Jews nationwide who remain supportive of the Democratic Party and a more nuanced relationship with Israel.

“The very, very limited set of people who do their politics simply through the lens of Israel — that small group is tilting more heavily Republican now,” he said, adding, “But it is dangerous for American politics as too many people do not understand that of the six million American Jews, this is only a handful.”

The deepening support for Israel among congressional Republicans reflects a significant shift for the party that has been playing out for several decades, said Geoffrey Kabaservice, a Republican Party historian.

“Israel did not traditionally represent that kind of emotional focus for any element of the Republican Party,” he said. “But the feeling now is that it is a winning issue, as it helps them to appear strong on foreign policy.”

Mr. Cotton declined to be interviewed, but his communications director, Caroline Rabbitt, said his aggressive support of Israel was based on longstanding beliefs, not any particular financial backer. “He knows Israel is one of the most important allies and a beacon of democracy in an otherwise perilous part of the world,” she said.

Over all, the most significant contributor by far to Republican supporters of Israel has been Sheldon Adelson, the casino magnate, who with his wife has invested at least $100 million in conservative causes over the last four years. A large chunk was spent on the 2012 presidential campaign, but Senate Republicans also benefited, and could soon again, particularly those considering a run for president.

The scope of the alliance was evident last month when Prime Minister Benjamin Netanyahu of Israel, at the invitation of Speaker John A. Boehner, spoke to a joint meeting of Congress, over Mr. Obama’s objections.

After the speech, some of the nation’s most important pro-Israel donors, including Mr. Adelson, gathered with more than a dozen Republican members of Congress at the nearby Capitol Hill Club. “It was a love fest,” said Kenneth J. Bialkin, a corporate lawyer and donor who attended.

The Senate floor has become another showcase for Republicans to demonstrate commitment to Israel. Senators Marco Rubio, who is expected to announce his presidential candidacy on April 13, and Mark S. Kirk, a moderate from Illinois, were among the Republicans who offered budget amendments related to Israel last month. Mr. Rubio blasted the White House for recent remarks critical of Mr. Netanyahu, calling them “outrageous.”

The shift has also meant the Republican Party today accepts little dissent on the topic of Israel, said Scott McConnell, a founding editor of The American Conservative, an outcome he believes is in part driven by the demands of financial supporters.

“Republicans interested in foreign policy used to understand that it was not in America’s national interest to ignore entirely Arab claims against Israel,” he said. “Now, there is a fanatical feeling of one-sidedness.”

Several Republican senators and champions of Israel rejected any suggestion that the intensity of the party’s recent support had anything to do with money.

“The Senate is the place where traditionally foreign policy has been a central issue because we have the unique responsibility to approve or disapprove treaties,” said Senator Rob Portman, Republican of Ohio, another major recipient of financial support from pro-Israel donors.

What is clear is that pro-Israel leaders have developed close relationships with some of the Republican champions like Mr. Cotton. It is highly unusual for a freshman senator to take a bold step like his Iran letter, and then persuade dozens of colleagues to endorse it. A spokesman for Mr. Cotton said the senator had written the letter himself, though Mr. Kristol said he had had a conversation with him about it.

“I know there has been all this fervent speculation that Tom Cotton and Bill Kristol and Sheldon Adelson were at some private room at the Venetian cooking this up,” said Noah Pollak, the executive director of the Emergency Committee for Israel, referring to Mr. Adelson’s casino in Las Vegas, where many prominent Republicans and Jewish leaders will gather this month. “But Tom is a smart guy and has a long record of thinking about the Middle East, and he is entrepreneurial. Tom wrote this letter.”

Mr. Graham said it was the tension between Mr. Netanyahu and the Obama administration that had highlighted the strength of the ties between Republicans and Israel. “But at the end of the day, the foundation of the U.S.-Israel relationship is bipartisan,” he said. “There is wide and deep Democratic support for the state of Israel. It’s just sort of getting left out of the story line here.”

Correction:April 5, 2015

An earlier version of this article misstated how Paul Singer, a hedge fund billionaire from New York, made a donation to Arkansas Horizon, an independent expenditure group supporting Senator Tom Cotton. He made the donation himself, not through a firm he runs. The article also misstated how Seth Klarman, a Boston-based billionaire, contributed to the group. He also made the donation himself, not through his investment firm.

Jennifer Steinhauer contributed reporting.

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